A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam of radiation in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The dimensions of the circuit patterns on the target portions may be very small. In order to use a patterning device on which the circuit patterns have larger dimensions, a projection system is used. This projection system reduces all circuit dimensions with the same factor. The projection system consists of several lenses or mirrors.
During engineering, the projection system is optimized as far as possible. However projection systems may be very difficult to produce and may contain manufacturing errors to a certain extent. For example, the powers of lenses may vary due to manufacturing tolerances, and the positions of the lenses in the projection system may also vary due to the manufacturing tolerances. Both of these variations may cause aberrations of the projection system, or change the power of the system to a non-optimal value.
Additionally, the projection system is optimized for a specified wavelength of projection beam radiation. However, in practice the wavelength of the beam of radiation passing through the projection system may vary, and because of dispersion within the lens elements, this may change the power of the projection system to a non-optimal value.
Finally the material of the lenses may decay because of the highly intense radiation of the projection beam that passes through them, modifying the transmission characteristics of the lenses.
Generally, the position of at least one lens element of the projection system is controlled by a mechanism which controls the system so that aberrations are minimized and magnification errors are corrected. The control mechanism works by adjusting positions of the lenses. However, in some cases, the settings of the control mechanism may drift over time, with the effect that lenses are wrongly moved to non-optimal positions.
The production of projection systems with mirrors may be subject to comparable difficulties.
In U.S. Pat. No. 5,631,731, the performance of the projection system is analyzed by patterning the beam of radiation with a test pattern and gathering position dependent information on the image of the test pattern. This image is formed in the focal plane of the projection system. The information is gathered by moving a slit through this image and measuring the amount of radiation that passes the slit. The slit is chosen with a small width to provide the highest resolution possible. A high resolution is necessary since the image is determined by the convolution of the test pattern and the performance of the projection system. The wider the slit, the lower the resolution, and the less accurate the performance can be calculated since the information will be smeared out. To gather information on projection systems operating close to the diffraction limits, the slit is chosen to have a width smaller than the wavelength of the projection beam radiation.
The radiation transmitted through the moving slit is measured by a photo detector, either directly or after the wavelength has been converted to a wavelength convenient for the photo detector. The output of the photo detector, combined with information regarding the position at which the measurement is obtained, gives a high resolution energy profile of the generated image. Energy profiles for slits with different directions are obtained for analysing performance of the projection system imaging lines with different directions. A deconvolution is applied on the profiles using a profile derived from a theoretical, perfect image of the reticle formed by a perfect lithographic apparatus, which perfect image is stored in software, to find the performance of the projection system.
The measurements may suffer from a dependency on the polarization state of the beam of radiation because the transmissions through the slits are polarization dependent. If polarization dependency causes a lower energy to be measured with the slit in a first direction than the energy measured with the slit in a second direction, this may be erroneously interpreted as a difference in projection system performance. U.S. Pat. No. 5,631,731 describes how measurement errors for different polarization states are suppressed by designing the slits so that the transmitted radiation is equal for the transmitted polarization directions, or by sequentially scanning two slits which transmit radiation with different polarization directions through the same test pattern image. Since the transmission of the two slits for their respective polarization directions may not be equal, a model is used to calculate the transmissions for the two slits. Both the transmissions and measured intensities are used to compensate for polarization effects. However, imperfections in the model lead to errors in the compensation for polarization effects.
In the event that energy ranges in the energy profiles for the first slit and the second slit differ significantly, the signal to noise ratio of the photo detector introduces additional measurement errors. These significant differences arise for instance when polarized radiation is present in the image.
To analyze polarization effects, the slit plate needs to contain slits with different orientations. The slits and their corresponding test patterns are positioned so that only the slit, or slits, of a given orientation are illuminated at a given time. To allow this to be done, the slits with different orientations are spatially separated (i.e. in different domains). Separate detectors can be used to measure the slits in the different domains. However, using a set of two detectors for two slits in different domains, will cause measurement errors because of the different behaviour of the detectors, such as sensitivity and signal to noise ratio.
If one detector is used instead of two detectors, there may be other disadvantages. The detector may be arranged to be capable of measuring the radiation from both spatially separated slits in the slit plate on different spatially separated domains of the detector. However, the spatially separated domains on the detector should respond equally to radiation falling onto them. It may be difficult and expensive to produce such a detector.
A further disadvantage is that the detector (or set of two detectors) and corresponding slit plates are large and therefore heavy. This is a disadvantage because the detector and slit plate are placed on the x-y-z positioning stage and space is scarce on the x-y-z positioning stage. Also, weight is unwanted on the x-y-z positioning stage in order to reduce the size of necessary drives and their power consumption and heat dissipation at height accelerations of the x-y-z positioning stage.
Instead of placing the photo detector under the slit plate, the radiation from the two slits may be collected by a light pipe (e.g. an optical fiber) so that only one domain on the detector is used for the radiation from the two slits with different orientations. In the light pipe, the collected radiation of the first slit propagates until a point where radiation collected from the second slit propagates. From this point to the detector, the propagation of radiation from the first slit and the second slit follows equal optical paths. Until this point, the optical paths are different.
Differences in losses during propagation in the different optical paths, may lead to measurement errors. A further disadvantage is that adding a light pipe to x-y-z positioning stage next to the slit plate and the detector, adds weight and will use space, and, as mentioned above, space is scarce on the x-y-z positioning stage. Also, weight is unwanted on the x-y-z positioning stage in order to reduce the size of necessary drives and their power consumption and heat dissipation at height accelerations of the x-y-z positioning stage.